That's cool.
Well, when you were editing Feature Settings, you were setting the Default Page path (the section is name Default Page, as you can see). And in applicationHost it was among locked attributes of httpErrors module. So, what I did was to unlock that specific attribute.
I don't know how safe is it, to leave it like this, you might want to check with others, but obviously, after setting your custom path, you can lock the attribute once again, if that's a problem.
Cheers.